Tube bending method

ABSTRACT

Large diameter thin-walled pipe is bent by a ram pushing the pipe forward axially and a bending arm forcing the forward end of the pipe to follow a curved path. A heater mounted in a plane passing through the center of the bend heats the pipe in a narrow bending zone to a temperature substantially reducing the yield strength of the metal. Heat input and cooling are adjusted around the periphery of the bend to provide uniform temperatures irrespective of changes in pipe wall thickness resulting from the bending. The flexure axis of the bend may be moved inward or outward along the bend radius by pulling or retarding with the pivotal bending means, or may be fixed by providing relatively cool spots in the heated band at the desired location of the flexure axis. Guide means are provided which may pre-deform the pipe laterally to compensate for lateral deformation during bending or may prevent deformation during bending. The portion to be bent may have a different diameter or thickness to compensate for changes in dimensions resulting from bending.

United States Patent Stuart [4 1 Sept. 2, 1975 l TUBE BENDING NETHOD[75] Inventor: James M. Stuart, Costa Mesa, Calif. [73] Assignee:Rollmet, Inc., Santa Ana, Calif.

[22] Filed: Apr. 1, 1974 [21 Appl. No.: 456,787

Primary Examiner-Lowell A. Larson Attorney, Agent, or Firm-Knobbe,Martens, Olson, Hubbard & Bear 1 5 7 ABSTRACT Large diameter thin walledpipe is bent by a ram pushing the pipe forward axially and a bending armforcing the forward end of the pipe to follow a curved path. A heatermounted in a plane passing through the center of the bend heats the pipein a narrow bending zone to a temperature substantially reducing theyield strength of the metal. Heat input and cooling are adjusted aroundthe periphery of the bend to provide uniform temperatures irrespectiveof changes in pipe wall thickness resulting from the bending. Theflexure axis of the bend may be moved inward or outward along the bendradius by pulling or retarding with the pivotal bending means, or may befixed by providing relatively cool spots in the heated band at thedesired location of the flexure axis. Guide means are provided which maypre-deform the pipe laterally to compensate for lateral deformationduring bending or may prevent deformation during bending. The portion tobe bent may have a different diameter or thickness to compensate forchanges in dimensions resulting from bending.

3 Claims, 14 Drawing Figures PATENTEUSEP' 21975 '3. 902344 SHEET 2 [IF 4f 5| 74/ i r PATENTEDSEP ms 3,902,344

snmsugg PATENTED SEP 2191s SHEET l [If 4 TUBE BENDING METHOD SUMMARY OFTHE INVENTION This invention relates to the forming of bends in largediameter pipehaving relatively thin walls. This application is relatedto my copending application for Method and Apparatus for Bending Pipe,"Ser. No. 447,886, filed Mar. 4, 1974, and the apparatus and methoddisclosed herein may also incorporate the fea' tures disclosed in thatapplication;

Forming bends in thin-walled tubing or pipes has been a very difficultprocedure because the walls tend to buckle or wrinkle on the inside ofthe bend when subjected to the substantial stress required for bending.This tendency is stronger, the thinner the wall thickness in relation tothe diameter of the pipe. Prior art commercial techniques oftensupported the pipe internally or externally to resist buckling. Thatrequires tooling which is extremely costly for large diameter pipe. Inaddition, the problem of extracting the finished part from the toolingseriously limits the complexity of the bends that can be formed.

It is known that pipe can be bent by pushing it along its axis from oneend while guiding the leading end about a radius. However, such atechnique is completely unsatisfactory for thin-walled pipe because thecompressive stress on the inside of the bend exceeds the column strengthof the pipe wall, and buckling results. US. Pat. No. 785,083, issued toBrinkman .describes such a proceduring using a wide heat zone to reducebending forces.

In order to overcome buckling, a narrow heat zone must be employed asdescribed by Hirayama et al in US. Pat. No. 3,368,377. The narrowness ofthe heated zone is of paramount importance as it is a result of thisnarrowness that the pipe is prevented from wrinkling or buckling. Theprinciple involved is that a column compressed at either end willthicken rather than buckle if the thickness of the column is more thanabout one half of its length. The metal at the inside of the bend in thehot zone may be regarded as a short column under compression. Sincebending occurs only in the heated zone, in this analogy the length ofthe column corresponds to the width of the heat zone and the thicknessof the column corresponds to the wall thickness.

The width of zone which may be permitted without buckling is dependenton the wall thickness of the pipe and to a lesser degree on the diameterof the pipe. For a pipe of exceptionally thin wall in relation to itsdiameter, the heated zone should be as narrow as twice the wallthickness. For thicker walls, or smaller diameters, the zone may widensomewhat. In the case of thickwalled pipes and solid bars the tendencytoward buckling is so limited that little attention need be paid to thewidth of the heat zone. In these cases heat is used merely to reduce thebending forces. In general, the zone should be as hot as possiblewithout microstructural deterioration of the metal.

A number of problems can arise when bending pipe heated in a narrow bandin accordance with prior art practices. For example, the pipe at theinside portion of the bend thickens under compression as the bendismade, while the outside portion thins under tension, resulting innonuniform wall thickness in the bent form if uniform blank material isused. Accordingly a uniformly distributed heat source tends to overheatthe thinner portions while underheating the thicker portions. Applicantovercomes this problem by applying greater or less heat or coolingaround the periphery of the bend depending on the local wall thickness.

Further, prior art processes provide no means for controlling oradjusting the amount of stretching or compression caused during bending.The bending moment causes compression in the inside or crotch of thebend, and stretching in the outside of the bend. In some applicationsthe range of possible bends will be limited by the tendency to formwrinkles in the areas in compression. In others the range will belimited by the inability of the metal to stretch without cracking in thestretched area of the bend. In accordance with one as pect of thisinvention the axial force applied by the ram pushing the pipe iscomplemented by a second axial force applied to the pipe through thepivotal arm. This complementary force may either assist or oppose theforward motion of the pipe through the heated zone. In the first case,the pipe is stretched along the arc of the bend, reducing the tendencytoward buckling in the bend crotch. In the second case, the pipe iscompressed along the arc of the bend and the amount of stretching andthe tendency to crack is reduced.

In the prior art the final bend wall thickness is prede termined by thebend geometry and the starting wall thickness. By means of thecomplementary force, in conjunction with the ram, the final wallthickness may be increased or reduced.

In addition, the location of the flexure axis may be controlled inaccordance with this invention by designing the heating means so as toprovide relatively cool spots in the heated annular band on oppositesides of the pipe at the desired location of the axis. As the coolerportion will not bend under the load causing bending in the heatedportion, compression will result below the cool spots and stretchingabove.

In accordance with another aspect of this invention, problems withdeformation of the pipe during bending are eliminated. Preferably, thisis accomplished by guide rollers bearing on the pipe upstream of theheated zone. In accordance with one embodiment, the rollers predeformthe pipe to compensate for the subsequent deformation during bending. Inaccordance with another embodiment, the rollers confine the pipedimensionally so as to prevent deformation.

Further in accordance with this invention, the portion of the pipe to bebent may, for example, be prethickened in the portion to form theoutside of the bend and prethinned in the portion to form the crotch ofthe bend so as to compensate for the thinning and thickening whichoccurs in bending. The diameter may also be preadjusted to compensatefor dimensional changes which occur during bending; all with the endresult of a bend portion having a diameter and wall thicknesssubstantially the same as the straight portions.

These and further features of this invention will be apparent from thefollowing description of a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of anapparatus constructed in accordance with this invention;

FIG. 2 is a side elevation of the apparatus of FIG. 1 showing a pipe inthe process of being bent;

FIG. 3 is a partial longitudinal section through the pipe and theapparatus;

FIG. 4 is a section taken generally along lines 44 of FIG. 3;

FIG. 5 is a section taken generally along lines 55 of FIG. 4; I I

FIG. 6 is a schematic sectionillustrating the forces buckling a pipe inprior art processes;

FIG. 7 is a schematic section illustrating the dimensional effect ofbending a pipe in accordance with this invention;

FIG. 8 is a longitudinal section through a portion of a modified form ofpipe prior to bending;

FIG. 9is a longitudinal section through the pipe portion of FIG. 8,after bending;

FIG. 10 is a cross-section through a portion of a modified form of pipeprior to bending;

FIG. 11 is a cross-section through the pipe portion of FIG. 10, afterbending;

FIG. 12 is a schematic representation of a pipe crosssection and astrain diagram showing the flexure axis approximately in the center ofthe pipe;

FIG. 13 is a schematic representation like FIG. 12, but showing theflexure axis lowered; and

FIG. 14 is a schematic representation like FIG. 12, but showing theflexure axis raised.

DESCRIPTION OF EXEMPLARY EMBODIMENT Referring to FIGS. 1 and 2, a pipe 2in which a bend is to be formed is shown in position on the apparatus ofthis invention. By way of example, a typical pipe would be of 304stainless steel of an overall diameter of about 16 inches and having awall thickness of about inch. The trailing end of the pipe is receivedin a collar 4 which is mounted on a ram 6. The ram includes a slide 8mounted by suitable guides such as the illustrated flange 10 and groove12 for sliding movement along a base 14. Suitable power means such as ahydraulic or pneumatic cylinder (not shown) is attached to the slide 8for pushing the pipe 2 axially in the direction of the arrow 16.

A bending arm 18 is removably but rigidly connected to the forward endof the pipe 2 so that the forward end of the pipe and the bending armmove as a unit. The bending arm is pivotally mounted on the base 14, forexample by a shaft 20, for pivotal movement about the shaft 20 which isthe desired center of curvature of the bend.

A ring 22 including roller type guides surrounds the pipe to guide thepipe in its movement forward, and for deformation control as will bedescribed below.

A heater ring 26 is mounted so as to be disposed around the pipe in aplane perpendicular to the longitudinal axis of the pipe and passingthrough the axis of bending 20. The heater ring may either be a gasburner as illustrated or an induction heater. If a gas burner is used,it is preferred to also provide an internal burner ring as described inmy copending application. Ser. No. 447,886, filed Mar. 4, 1974, referredto above. The internal burner has been omitted in these drawings inorder to simplify the disclosure.

Referring to FIGS. 3 and 4, the exemplary burner ring has a pluralityofjets 28 connected to a gas supply line 29 and disposed around thecircumference of the pipe 2 so as to heat the pipe to a temperature atwhich its yield strength is substantially reduced. Forsteel thispreferably is a temperature in the range of about 1,600F. to 2,000F.,and preferably about l,80()F. The range of temperatures could bebroader, but there is no significant-change in' the yield strength attemperatures below l,200F., and metallurgical deterioration begins attemperatures in excess of 2,200F.

A pair of quench rings 30, 32 are also disposed around the pipe 2 onopposite sides of the burner ring 26. Each quench ring includes a setofjets; those of the first ring 30 being directed slightly forward ofthe burner ring and those of the second ring 32 being di rected slightlyrearward of the burner ring. The quench ring jets are connected to asource of coolant such as air, or preferably water, or a combination ofthe two, to cool the pipe 2 forward and rearward of the burner 26 andrestrict the heated zone to a narrow band.

In operation, the ram 6 is urged forward by its power source to move thepipe 2 longitudinally of its axis in the direction of the arrow 16.Since the forward end of the pipe can move forward only arcuately withthe bending arm 18, the path of the forward end of the pipe necessarilyis through an arc having its center of curvature at the axis 20 of thebending arm. Accordingly, a torque or bending moment is applied to thepipe 2 tending to bend the pipe about the axis 20.

Referring now to FIG. 6, which is a schematic longitudinal sectionthrough a portion of a representative pipe 2, it will be evident thatthe bending moment applied to the pipe applies compressive forces 54 tothe inside portion of the bend and tensile forces 56 to the outsideportion of the bend. The compressive forces tend to shorten the pipe onthe inside portion of the bend and the tensile forces tend to lengthenthe pipe on the outside portion of the bend from its initial shape asshown in the dashed lines.

If the thickness of the pipe wall is relatively small in relation to thediameter of the pipe as is the case with the pipes for which thisinvention finds it primary application, the compressive forces 54generated on the inside of the bend in order to cause bending at normaltemperatures tend to form buckles or wrinkles 58 in the metal. Thistendency is stronger the thinner the wall thickness in relation to thediameter of the pipe, and makes impractical the bending of largediameter thinwalled pipe. It is for that reason that the narrow heatzone is used.

The burners 26, 34 are designed to generate sufficient heat to raise thetemperature of the pipe 2 in the narrow heated zone to above apreselected temperature at which the yield strength is substantiallyreduced, e.g. l,800F. With the reduced yield strength, the requiredbending moment, and therefore the resulting compressive stress is less.

A constant preselected pressure is kept on the ram 6 which is sufficientto cause bending stresses in the pipe 2 equal to the yield strength ofthe metal at the preselected temperature. Since longitudinal movement ofthe pipe can occur only as the pipe bends, the pipe does not move untilthe temperature in the heated zone reaches the preselected temperature.At that time the compression and tension in the heated zone equals orexceeds the yield strength of the metal and the metal begins to bend,thereby permitting the pipe 2 to advance under pressure of the ram 6. Asthe pipe'advances, new cold metal is brought into the heated zone andthe bending process continues when the new portion is heated to theappropriate temperature. The temperatureofthe heated zone is notcontrolled directly, but is controlled by the ram pressure setting. Thetem perature increasesuntil the metal yields, under the applied force ofthe ram 6. As the pipe 2.bends, it passes slowly through the burners 26,34 and hot metal is displaced by cold metal. The speed with whichythenew metal is heated to the forming temperature, which controls the rateof advance of the. pipe 2, is dependent on the amount of excess heatavailable. Thus, .the apparatus is self-regulating in that no movementwill occur until the desired temperature is achieved since no bendingwill occur until the-metals .yield strength is reached. 1

The bend resulting from thisinvention is typified by FIG. 7 which showsa straight pipe portion 2a having uniform wall thickness entering theheated zone -57. In the heated zone, the bending moment causes stressexceeding the yield strength of the metal. Accordingly, the wall portionat the outside of the bend abruptly thins at 60 and the wall portion atthe inside of the bend abruptly thickens at 62. i

The thicker wall portions dissipate more'heat than the thinner portions.Thus, if heat and cooling are applied uniformly around the circumferenceof the pipe 2, and on either side of the burner, the thicker portionsgenerally will not attain the desired temperatures, while the thinnerportions will be overheated. Accordingly, the amount of heating orcooling applied preferably is not uniform around the circumference andon either side of the burner.

The amount of heat dissipated in the metal is strongly dependent on theproximity of the cooling jets to the heat source. The closer the jetsthe greater the heat dissipation, and vice versa. Thus, the unequal heatdissipation resulting from dimensional changes may be compensated for bymoving the quench sprays closer or further from the heat ring.

Alternatively, nonuniform heat input around the pipe periphery may beaccomplished. For example, with a gas burner heater, the gas jets may belarger or more closely spaced at the crotch than at the outside of thebend. With an induction heater the heater may be spaced further from thepipe at the outside of the bend than at the crotch, or a nonuniformlamination density may be used.

In some instances, for example where composite material is used, thetemperature may deliberately be made nonuniform to suit the propertiesof each material, by employing the above methods.

Referring now to FIG. 12, a schematic strain diagram 63 represents thestrain in the wall of pipe 2 in the heated bending zone. This strainvaries substantially linearly from maximum stretching T at the top 60 tomaximum compression C, at the bottom 62 with the flexure axis 64 (at thepoint where the strain is zero) occurring approximately midway. Ifwrinkling is likely to occur, it may be desirable to lower the flexureaxis 64 as shown in FIG. 13 so that the maximum compression C is lessthan the compression C In that instance the maximum stretching T will begreater than T but that may be acceptable, or even desirable with somepipe dimensions or materials.

Similarly if cracking is likely to occur it may be desirable to raisethe flexure axis 64 as shown in FIG. 14. Accordingly, the maximumstretching T, will be less than the stretching T although there will bea related increase in the maximum compression C over C Such variation ofthe flexure axis may, for example, be accomplished with a hydraulic orpneumatic cylinder and pulley as shown in FIGS. 1 and 2. A pulley 66 ismounted on the axis 20 and fixed for movement with the'arm 18. A firstcylinder 65 is suitably mounted on the base 14*and has its piston rod 61connected to a wire rope 67 which extends partially around the pulley 66in'a clockwise direction 'andis suitably fastened to thepulley, forexample at 69. A second cylinder 71 is similarly mounted on the base andconnected to a wire rope 73 which extends counterclockwise partiallyaround the pulley 66 and is connected at 69, for examl. J I r To lowerthe flexure axis the cylinder 65 is actuated to pull the bending arm 18thereby applying added tension to the pipe. To raise the flexure axisthe cylinder 71 is activated to resist forward pivoting of the arm 18thereby applying additional compression to the pipe. In either event theamount of force applied by the cylinder will determine the distance theflexure axis is raised or lowered.

Further, the location of the flexure axis may be controlled to someextent by designing the heater so as to provide relatively cooler spotsin the annular heated band on opposite sides of the pipe at the desiredlocation of the flexure axisf The cooler spots will have a higher yieldpoint and therefore will serve as a pivotal point about which the pipewill bend. That is, portions above the cooler spots, in orientation ofthe exemplary configuration, will expand and therefore be in tension,whereas portions below the cooler spots will contract and therefore bein compression.

In order to achieve the cooler spots, the gas burner can be designed tohave no jets at the desired flexure axis location, or the inductionheater can be designed to have breaks in its are at those locations, orthe quench rings can be arranged to provide added coolant directly onthose spots.

In some instances the bending of the pipe may cause lateral deformation,that is the pipe may increase in diameter in its dimension perpendicularto the plane of the bend, and decrease in diameter in its dimension inthe plane of the bend. This can be prevented with the guide 22 shown inFIGS. 1 and 5. The guide includes a ring 22 suitably supported from thebase 14 by brackets to surround the pipe 2. A plurality of rollers 72are mounted on the ring and extend radially inwardly toward the pipe.Each roller is mounted on a threaded stud 74 which provided forindependent adjustment of the radial distance from the center of thering to each roller. The rollers are adjusted to contact the peripheryof the pipe and guide it in its forward movement. Further, the rollerscan be adjusted to put sufficient radial pressure on the pipe toconstrain it against lateral deformation. Alternatively, the siderollers 72a and 72b can be adjusted inward to a spacing less than thenorml outside diameter of the pipe and the top roller 72c and bottomroller 72d adjusted outward to a spacing greater than the normal outsidediameter of the pipe to predeform the pipe sufficiently to allow for thedeformation which will take place in bending, thereby resulting in abend portion of the diameter and shape desired.

As shown in exaggerated form in FIGS. 8 and 9, in some instances thepipe 2 to be bent may first be thickened in the area to be bent 76.Thus, when the pipe is bent the outside portion of the bend 62 will havea wall thickness equal to that of the straight pipe portion 77. This isdesirable where specifications require a minimum wall thickness.

As shown in FIG. 10, in some instances the pipe 2 to be bent may firstbe made eccentric with thin wall in the area to become the bend crotch62, and with thick wall in the outer bend region 60. Thus, as shown inFIG. 11, the eccentricity may be designed so that the bent pipe willhave uniform wall thickness equal to that of the straight pipe portion.This is desirable where specifications require a uniform wall thickness.

Further, the pipe to be bent may first be expanded in diameter in thearea to be bent. This can be designed to result in an inside pipediameter in the bent portion equal to that in the straight portion.

What is claimed is: l. A device for bending pipe comprising: means foradvancing a pipe past a heating station; heating means at the heatingstation for heating a narrow annular band of the pipe to a temperaturesufficient to substantially reduce its yield point; means for applying abending moment to the heated portion of the pipe; means varying the heataround the periphery of the band; said heating means including coolingmeans for cooling the pipe forward and rearward of said heating means toconfine the heated area to a narrow band; and

said means varying the heat comprising means varying the spacing betweensaid forward and rearward cooling means around the periphery.

2. A device for-bending pipe comprising:

means for advancing a pipe past a heating station;

heating means at the heating station for heating a narrow annular bandof the pipe to a temperature sufficient to substantially reduce itsyield point;

means for applying a bending moment to the'heated portion of the pipe;and

said heating means including means for interrupting the annular heatedband of pipe by cooler portions on opposite sides of the pipe atopposite ends of the desired flexure axis to control location of theflexure axis.

3. A method of bending a thin-walled pipe of large diameter comprising:

heating a narrow band of the pipe to a temperature substantiallyreducing its yield point;

applying a bending moment to the pipe portion in the narrow band;

progressively moving the narrow band along the pipe as the portion inthe band bends; and

providing relatively cool spots in said band on opposite ends of thedesired flexure axis.

1. A device for bending pipe comprising: means for advancing a pipe pasta heating station; heating means at the heating station for heating anarrow annular band of the pipe to a temperature sufficient tosubstantially reduce its yield point; means for applying a bendingmoment to the heated portion of the pipe; means varying the heat aroundthe periphery of the band; said heating means including cooling meansfor cooling the pipe forward and rearward of said heating means toconfine the heated area to a narrow band; and said means varying theheat comprising means varying the spacing between said forward andrearward cooling means around the periphery.
 2. A device for bendingpipe comprising: means for advancing a pipe past a heating station;heating means at the heating station for heating a narrow annular bandof the pipe to a temperature sufficient to substantially reduce itsyield point; means for applying a bending moment to the heated Portionof the pipe; and said heating means including means for interrupting theannular heated band of pipe by cooler portions on opposite sides of thepipe at opposite ends of the desired flexure axis to control location ofthe flexure axis.
 3. A method of bending a thin-walled pipe of largediameter comprising: heating a narrow band of the pipe to a temperaturesubstantially reducing its yield point; applying a bending moment to thepipe portion in the narrow band; progressively moving the narrow bandalong the pipe as the portion in the band bends; and providingrelatively cool spots in said band on opposite ends of the desiredflexure axis.